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Mars Companions

StarDate:

December 1, 2013

The moons of Mars are scrawny little things — chunks of rock that are only a few miles across. But they’re not the only cosmic flotsam that accompanies the Red Planet. At least seven asteroids share its orbit around the Sun.

Collectively, the asteroids are known as “Trojans.” One of them orbits 60 degrees ahead of Mars, while the others form a clump 60 degrees behind the planet. The asteroids are held in place by a gravitational balance between Mars and the Sun. Mars is the only inner planet known to have Trojan companions.

The first was discovered in 1990, and was named Eureka. Two others were discovered soon afterwards. The last four were identified as Trojans only recently, by a researcher in Northern Ireland. He combed through records of asteroid orbits and found that four of them matched the orbit of Mars.

He also found that all but one of the asteroids in the clump that trails Mars are small, and they’re bunched up around Eureka. That suggests that they’re the remnants of a larger asteroid that was shattered by a collision. The impact wasn’t strong enough to kick them out of position, though — it left them in Mars’s orbit, giving the Red Planet a few small, distant companions.

And Mars has a couple of bright companions in our sky right now. The planet looks like a bright orange star well up in the southeast at first light. The star Regulus is to its upper right, with Spica about the same distance to its lower left.

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